Thread: Big Cam Really Low Vacuum?

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Hi All,

I've been having an intermittent bogging problem with the Firebird while racing so I've been looking to see if I can tune it out (I know the real solution to this problem is a looser converter but I'm trying to get by for now).

First step was going to make sure idle circuits were tuned then go from there with the power valve and maybe larger pumps.

So I hook up my vacuum gauge to the rear vacuum port on the carb (which I'm pretty sure is manifold vac) and found I was only at 4-5 inches at 800rpm idle (whether in gear or not). Vac didn't change hardly at all until I leaned out enough to kill the motor.

4-5" seems incredibly low (I was expecting at least 8-10ish). Should I expect vacuum to be this low with a large cam?

I'm running a 427++ BBC with:
.630/.630 lift
255/264@.050 duration
110 LSA and 104 ICL solid roller

and 850cfm Holley double pump 4150 race (0-9380 list) carb.
I'm running a 6.5 power valve, so this also has me concerned that it will always be open at idle

If that's all the vac I get, it doesn't bother me much because I don't need it for brakes or trans just wondering if this is abnormal and I should look closer for leaks. BTW: vac comes up into the teens when I rev it in neutral.

I'll start another thread on the bogging problem...

Thanks in advance,

-Chris

Originally Posted by skids72

Hi All,

I've been having an intermittent bogging problem with the Firebird while racing so I've been looking to see if I can tune it out (I know the real solution to this problem is a looser converter but I'm trying to get by for now).

First step was going to make sure idle circuits were tuned then go from there with the power valve and maybe larger pumps.

So I hook up my vacuum gauge to the rear vacuum port on the carb (which I'm pretty sure is manifold vac) and found I was only at 4-5 inches at 800rpm idle (whether in gear or not). Vac didn't change hardly at all until I leaned out enough to kill the motor.

4-5" seems incredibly low (I was expecting at least 8-10ish). Should I expect vacuum to be this low with a large cam?

I'm running a 427++ BBC with:
.630/.630 lift
255/264@.050 duration
110 LSA and 104 ICL solid roller

and 850cfm Holley double pump 4150 race (0-9380 list) carb.
I'm running a 6.5 power valve, so this also has me concerned that it will always be open at idle

If that's all the vac I get, it doesn't bother me much because I don't need it for brakes or trans just wondering if this is abnormal and I should look closer for leaks. BTW: vac comes up into the teens when I rev it in neutral.

I'll start another thread on the bogging problem...

Thanks in advance,

-Chris

800 rpm idle?? with a cam this size??
I doubt it will even idle at this speed with 255 at .050 in a 427.
Try turning up your idle speed to 1,000 to 1,100 rpm's.
With a cam that size I would run a 3.5 power valve.

Thanks, Erik... Yeah 800 is pretty low but it runs like a top there (albeit quite lumpy ). I certainly can crank the idle up a few hundred... otherwise, 5" can be considered "normal" with this cam? I'll measure again at a higher idle...

Thanks again,

Chris

Originally Posted by skids72

Thanks, Erik... Yeah 800 is pretty low but it runs like a top there (albeit quite lumpy ). I certainly can crank the idle up a few hundred... otherwise, 5" can be considered "normal" with this cam? I'll measure again at a higher idle...

Thanks again,

Chris

Yes,I would say 5-7 would be the normal depending on where you have the cam installed.
That cam you have in there is really a "Pro-Street"/Strip cam.
I would think by 4,000 it really pulls very hard.

Yes indeed and that's what it's for... pulls to 7000 but it's kind of a dog in traffic. 12 flat in the 1/4 at this altitude on 91 octane but I think it has close to another .5 second to be tuned out (with real gas ).

Originally Posted by skids72

Yes indeed and that's what it's for... pulls to 7000 but it's kind of a dog in traffic. 12 flat in the 1/4 at this altitude on 91 octane but I think it has close to another .5 second to be tuned out (with real gas ).

I would guess at sea level and good tuning you would have a 11.30 car.

Originally Posted by erik erikson

I would guess at sea level and good tuning you would have a 11.30 car.

Should run there abouts at sea level... Was hoping to qualify for the trip to Topeka this year but it's not looking so good at this point... need to be in the top 6 for points and I've gone three races without winning a round so it doesn't look like I'll make it. Would be a great opportunity to see how she does with some air...

ive got a problem with low vac pressure, it does not effect the performance of the motor, but the vac on the brake booster. i am going to try an E VAC system, i hear they are around 80 bucks, but have not located one yet.

Originally Posted by mooneye777

ive got a problem with low vac pressure, it does not effect the performance of the motor, but the vac on the brake booster. i am going to try an E VAC system, i hear they are around 80 bucks, but have not located one yet.

Hey Mooneye, you talking about one of these. I found this at Summit for $32



Fortunately I don't need one for the old manual drum brakes...

Originally Posted by skids72

Thanks, Erik... Yeah 800 is pretty low but it runs like a top there (albeit quite lumpy ). I certainly can crank the idle up a few hundred... otherwise, 5" can be considered "normal" with this cam? I'll measure again at a higher idle...

Thanks again,

Chris

Man 800RPM.....I would have this motor idle set to 1,100+.....just to aid in oiling. I am sure it sounds great loping away at 800rpm....but I am sure watching your oil pressure at idle after 15 minutes of running, is begging for a higher idle.....just my 2 cents.

I use to have a 454 with a giant bump stick. And at start up I had 60psi of oil pressure. After cruising around for 15-20 minutes the oil was hot and thin, and my lumpy idle gave me 40psi.....that was a little low, so the idle came back up to about 1000.

hey, Terror, thanks for the reply...

I get 30psi at 800 idle when everything is on the hot side, which I think is plenty of oil... I've turned it up to 1100 since this post but vacuum is still in the 4-5" range... I'm still running 6.5" power valves for now and I will be swapping to 4.5" in the near future... I keep a close eye on the plugs and haven't had any fouling problems yet... I should be at 2.5-3.5" power valve but I don't want to risk the lean out on the big end of the strip. I'm probably close to the point I ought to plug the PV's and jet it up, as Pat McCarthy suggested but I'm reluctant to do that for a still street driven car...

Chris

I am running a 4.5 for the exact same reason....I am not worried about that 1/10th of a second I could gain my leaning my motor out just a bit more....I would rather be running fat, than burning lean anyday.

Keep us updated on how she likes the new PV

Originally Posted by 4speedTerror

but I am sure watching your oil pressure at idle after 15 minutes of running, is begging for a higher idle.....

Not to take this too far OT, but could you explain that one to me? I don't doubt it, since I've witnessed it in my own car, but why does lumpy cam drop oil pressure at idle? Is it simply because of the drop in vacuum? I do know this: on my own car taking it to a guy who knows what he's doing with the exhaust (just had a temp exhaust on it) smoothed things out quite a bit. Don't know my cam specs, friggin builder won't tell me like it's some secret.

Originally Posted by marlinspike

Not to take this too far OT, but could you explain that one to me? I don't doubt it, since I've witnessed it in my own car, but why does lumpy cam drop oil pressure at idle? Is it simply because of the drop in vacuum? I do know this: on my own car taking it to a guy who knows what he's doing with the exhaust (just had a temp exhaust on it) smoothed things out quite a bit. Don't know my cam specs, friggin builder won't tell me like it's some secret.

ok something you said REALLY BOTHERS ME!!!! Your engine guy should ALWAYS give you a cam card with all the specs of your cam. What if you need new pushrods? How are you going to know what you need. And that goes for EVERYTHING in your valve train. So first of all I would slap him around and DEMAND the cam specs. Even if he grinds his own custom cams (which i HIGHLY HIGHLY doubt)

Ok and the idle question is simple.....If your idle is already low at first start up (cold start up in the mornings), then the longer you drive the car, the hotter the motor gets...which makes your oil thinner and thinner. So if you have had an idle of 700rmp in the morning and your oil pressure was 50psi, by time your motor is nice and hot, your oil is much thinner than in the morning. So now of course a thinner liquid will move faster. So your oil pressure falls. And for me a low oil pressure, and an aggressive cam makes me nervouse. I have oil restrictors and all that, but anything that is "splash" lubricated, I am going to make sure it is getting "splashed" very well...lol

Originally Posted by 4speedTerror

ok something you said REALLY BOTHERS ME!!!! Your engine guy should ALWAYS give you a cam card with all the specs of your cam. What if you need new pushrods? How are you going to know what you need. And that goes for EVERYTHING in your valve train. So first of all I would slap him around and DEMAND the cam specs. Even if he grinds his own custom cams (which i HIGHLY HIGHLY doubt)

Ok and the idle question is simple.....If your idle is already low at first start up (cold start up in the mornings), then the longer you drive the car, the hotter the motor gets...which makes your oil thinner and thinner. So if you have had an idle of 700rmp in the morning and your oil pressure was 50psi, by time your motor is nice and hot, your oil is much thinner than in the morning. So now of course a thinner liquid will move faster. So your oil pressure falls. And for me a low oil pressure, and an aggressive cam makes me nervouse. I have oil restrictors and all that, but anything that is "splash" lubricated, I am going to make sure it is getting "splashed" very well...lol

Well, it's an overhead cam, so no pushrods. And this engine doesn't rely on splash lubrication (it's a hot rodded Mercedes...yes, I know, I'm nuts).

The cams were custom ground...I suppose when I need a replacement cam I can just stick it in a cam doctor, no? He did kinda tick me off with that...as well as some other things he did.